7.15: Local Winds

Which large rainstorms are crucial for people in some parts of the world?

India, Pakistan and other nations in southern Asia are great places to go to see monsoon rains. The region depends on the water they bring. Parts of the Southwestern United States also receive monsoon rains. These rains break the summer heat. They also provide water to desert plants. With winter rains and summer rains, these deserts are more lush than many.

Local Winds

Local winds are winds that blow over a limited area. Local winds blow between small low and high pressure systems. They are influenced by local geography. Nearness to an ocean, lake or mountain range can affect local winds. Some examples are found below. Local winds can affect the weather and climate of a region.

Land and Sea Breezes

Ocean water is slower to warm up and cool down than land. So the sea surface is cooler than the land in the daytime. It is also cooler than the land in the summer. The opposite is also true. The water stays warmer than the land during the night and the winter. These differences in heating cause local winds known as land and sea breezes. Land and sea breezes are illustrated in
Figure
below
.

A
sea breeze
blows from sea to land during the day or in summer. That’s when air over the land is warmer than air over the water. The warm air rises. Cool air from over the water flows in to take its place.

A
land breeze
blows from land to sea during the night or in winter. That’s when air over the water is warmer than air over the land. The warm air rises. Cool air from the land flows out to take its place.

Land and sea breezes blow because of daily differences in heating.

Monsoons

Monsoons
are like land and sea breezes, but on a larger scale. They occur because of seasonal changes in the temperature of land and water. In the winter, they blow from land to water. In the summer, they blow from water to land. In regions that experience monsoons, the seawater offshore is extremely warm. The hot air absorbs a lot of the moisture and carries it over the land. Summer monsoons bring heavy rains on land. Monsoons occur in several places around the globe. The most important monsoon in the world is in southern Asia, as shown in
Figure
below
. These monsoons are important because they carry water to the many people who live there.

Monsoons blow over southern Asia.

Mountain and Valley Breezes

The air on a mountain slope warms more than the air over the nearby valley. The warm air rises and brings cool air up from below. This is a
valley breeze
. At night the mountain slope cools more than the air over the valley. The air flows downhill creating a
mountain breeze
.

Katabatic Winds

Katabatic winds
move the same way as mountain and valley breezes. However, they are much stronger. Katabatic winds form over a high plateau that is surrounded by mountains. In winter, the plateau grows cold. Air sinks through the gaps in the mountains. Over Antarctica and Greenland, these winds are frigid.

Chinook Winds

Chinook winds
occur when air is forced over a mountain range. Warm air rises over the Sierra Nevada in California, for example, because it is pushed eastward by the westerly winds. The air cools as it rises and precipitates. The air is now dry. It sinks down the far side of the mountains and may create strong winds. These Chinook winds are relatively warm. If there is snow, the winds may melt it quickly. The dry sinking air creates a
rainshadow effect
. Rainshadow effect is responsible for many of the world's deserts.

As air rises over a mountain it cools and loses moisture. The air warms by compression on the leeward side. The resulting warm and dry winds are Chinook winds. The leeward side of the mountain experiences rainshadow effect.

Santa Ana Winds

Santa Ana winds
are responsible for many large fires in Southern California. The Santa Ana winds arrive at the end of California’s long summer drought season. Air east of the Sierra Nevada Mountains cools in late fall. This creates a high pressure zone. The air is then forced downhill through the deserts of the Southwest. It blows westward toward the ocean. The air is blocked by the San Gabriel and San Bernardino Mountains. So it is funneled rapidly through the mountain passes. If a fire starts, it spreads quickly. The result is large-scale devastation.

The winds are especially fast through Santa Ana Canyon, for which they are named. Santa Ana winds blow dust and smoke westward over the Pacific from Southern California.

In October 2007, Santa Ana winds fueled many fires that together burned 426,000 acres of wild land and more than 1,500 homes in Southern California.

Desert Winds

High summer temperatures on the desert create high winds and monsoon storms. Strong winds in the desert can pick up dust and blow it around. (
Figure
below
). A dust storm known as a
haboob
forms in the downdrafts on the front of a thunderstorm.

A haboob in the Phoenix metropolitan area, Arizona.

Vocabulary

Chinook wind
: Winds that form when low pressure draws air over a mountain range.

haboob
: Desert sandstorms that form in the downdrafts of a thunderstorm.

katabatic wind
: Winds that move down a slope.

land breeze
: A wind that blows from land to sea in winter when the ocean is warmer than the land.

monsoon
: Hot land draws cool air off a nearby sea creating large winds and often rain.

mountain breeze
: A wind that blows from a mountain to a valley at night when mountain air is cooler.

rainshadow effect
: A location of little rain on the leeward side of a mountain range due to descending air.

Santa Ana winds
: Hot winds that blow east to west into Southern California in fall and winter.

sea breeze
: A wind that blows from sea to land in summer when the land is warmer than the ocean.

valley breeze
: An uphill airflow.

Summary

Local winds blow between high pressure and low pressure areas.

Winds blow up and down slope, on and off land and sea, through deserts or over mountain passes.

Some local winds are well known in an area: haboobs in the desert, Santa Ana winds in Southern California, Chinook winds in Colorado.